In U.S. Pat. Nos. 3,626,053 and 3,621,092, processes are described for molding fiberglass reinforced thermoplastic sheets utilizing presses. In the processes described in both patents, a reinforcing mat, typically formed of glass fibers, is utilized to reinforce thermoplastic resin in sheet form. The mat reinforced sheets are then stamped into shaped parts utilizing a press. Prior to placing the sheets in the press for stamping into shaped parts, however, the sheets must be heated to a temperature sufficient to render the resin of the sheet molten or flowable while maintaining the temperature of the sheet below the decomposition temperatures of the thermoplastic resin used to prepare the sheet. The heating systems described in both of these patents involve infrared ovens.
Various other patents have issued which relate to fiber reinforced thermoplastic sheet products such as those described in the aforementioned patents. Exemplary of some of these other patents are U.S. Pat. Nos. 3,664,909, 3,684,645 and 4,335,176. In all of these patents the product described is suitable for use in stamping or compression molding operation. In utilizing any of the fiber reinforced thermoplastic products described in these patents, the fiber reinforced thermoplastic resin sheet product is first heated to bring it to a temperature sufficient to render the resin component of the sheet flowable or molten. The heated sheet, while the resin is still in the flowable state, is then placed on a mold in a suitable press such as a hydraulic or mechanical press and pressure is applied to stamp or mold the sheet into a shaped part. As described in the aforementioned patents, rendering the resin molten prior to molding the fiber reinforced sheet, permits the resin to flow during molding and the reinforcement flows with the resin. This provides a shaped part which has reinforcement uniformly distributed throughout.
In preparing the reinforced thermoplastic resin sheets for the compression molding processes utilized in the art, recourse has been had to the utilization of infrared ovens for the heating of the sheets. It has also been common practice to employ an oven containing a standard cable conveyor system which oven is provided with opposing infrared heaters on the top and the bottom of the oven facing each other. The sheets are placed on the cable conveyor and moved through the oven or held there in a stationary position while heat is applied from the infrared heaters to render the resin contained in the reinforced resin sheets molten. Once the resin is molten the sheets containing the molten resin can then be placed on a cold mold in a hydraulic press. The press is closed quickly to provide for the reinforcement and resin to flow and fill the mold. The resin cools and solidifies as the part is stamped or molded resulting in a shaped product.
While infrared heating of fiber reinforced thermoplastic blanks or sheets has been the rule in industry to date, such heating does present certain disadvantages with respect to providing an efficient process. Infrared ovens are by nature, in the environment utilized to heat thermoplastic resin sheets, inefficient. In the heating of fiber reinforced thermoplastic sheets it has been found in practice that the reflectors or heaters become quite dirty due to the release of dust into the atmosphere usually from the edges of the reinforced thermoplastic resin sheets passed through the ovens. This dust which settles on the reflectors contaminating their surfaces require substantial percentage increases in the power fed to the heaters over time to compensate for losses in efficiency caused by this surface contamination. Increased heating of the infrared heaters also has the disadvantage of requiring excessive amounts of power to be utilized in heating a given sized sheet thereby increasing the cost of the production process. Further, by utilizing the infrared heaters at high energy levels continuously, the life of the heaters is substantially reduced.
In systems using black faced (infrared) heating elements positioned in ceramics, the same problem exists due to environmental contamination occurring on the ceramic surfaces. Again, increased power is required to overcome the loss of efficiency created by the deposition of a foreign material on the surface of the ceramic. Another disadvantage of the infrared system is the operational preferences attributable to individual operators in staging the heating of sheets passing through such ovens. Thus, it is common practice in the heating of thermoplastic resin blanks reinforced with fibers to pass them through the infrared oven in stages. The first stage causes considerable swelling of the sheets since most of them are reinforced with mats which have been compressed during the formation of the sheet itself. These mats have a tendency to swell the sheet once the resin reaches a flowable state during the heating operation, because the compressive forces in the mat are released once the sheet resin is no longer a rigid solid.
Thus, the sheets generally accept large quantities of heat at the inception of their passage through the ovens in the presence of infrared heaters operating at temperatures sufficient to cause the resin to melt. The sheet then, after swelling, is subjected to less severe temperature regimes but must soak in some heat in order to insure that the center of the blank receives sufficient heat to cause all of the resin in the blank to become flowable. It is in these latter stages of the heating, as the sheet passes through the oven and is subjected to stops that operators make the ultimate decision on how much power to input to that section of the oven. If too much heating occurs in these latter stages, the resin itself can deteriorate.
Finally, it is a further disadvantage of infrared ovens that the ovens with opposing infrared heaters (top and bottom) transferring heat from their surfaces to the upper and lower surfaces of a sheet moving on cable conveyors through heating ovens, are incapable of heating several layers of sheets passing through on multiple conveyors positioned one above the other, in the same oven. In any such arrangement, the bottom heaters would be prevented from heating the bottom surface of an upper row of sheets passing through a conveyor located immediately below them. This seriously limits oven capacity to two banks of heaters, one on the top and one on the bottom and a single conveyor passing through the oven so that only the sheets on one conveyor can be treated at one time.
Thus a need exists to improve the heating cycle for fiber reinforced thermoplastic sheet materials that are to be utilized in compression molding systems where heated sheets are placed in a cold mold and pressure is applied to shape those sheets into a formed part.